Devices and methods of this type serve the purpose, for example, of determining the imaging quality and/or image errors of high-resolution optical imaging systems interferometrically with high precision. An important field of application is the corresponding measurement of projection objectives in microlithography exposure machines for semiconductor component patterning. Interferometry techniques used for this purpose are shearing interferometry, by means of which the wavefront measurement device disclosed in Laid-open Patent Application DE 101 09 929 A1, for example, operates, and point diffraction interferometry. In this case, the device can be integrated in the system in which the imaging system is used in its normal operation, and it can use for measurement the same radiation of a radiation source present in the system as it is used in normal operation of the imaging system. In this case, the interferometer is denoted as an operational interferometer or OI device.
It is known in the case of these phase-shifting interferometry techniques for wavefront measurement that the phase-shifting structure, for example a diffraction grating, to be arranged on the image side, with a one-dimensional or two-dimensional diffraction grating structure, or a so-called coherence mask, to be arranged on the object side, with a one-dimensional or two-dimensional coherence mask structure, is moved laterally relative to the optical imaging system to be measured, in order to determine the spatial derivative of the measured wavefront in the relevant lateral direction, from which it is then possible to obtain image error information relating to the imaging system, in particular spatially resolved image error information relating to the entire pupil of the imaging system, typically in the form of so-called Zernike coefficients. Here, the designation “one- or two-dimensional” means structures which are periodic in one or in two non-parallel directions, and consequently lead in the diffraction diagram to the diffraction patterns in one or in two non-parallel directions.
For this purpose, for example, spatial derivatives in two mutually orthogonal directions such as the x- and y-directions of an xyz coordinate system with a z-axis pointing in the direction of the optical axis of the system are determined by using a two-dimensional coherence mask to be arranged on the object side, and a two-dimensional diffraction grating structure corresponding thereto. In addition to the stepwise, relatively slow lateral displacement, for example of the diffraction grating structure for the purpose of effecting the phase shift in the direction in which the spatial derivative of the interferogram or of the wavefront is to be measured, for example in the x-direction, it is preferred to provide an in contrast much faster lateral movement of the phase-shifting structure in the direction perpendicular thereto, such as the y-direction, in order to suppress effects by interference between undesired diffraction orders in this orthogonal direction. The interferogram image recorded by the detector element on the detection plane during this fast movement is integrated such that the undesired interference is averaged out as far as possible.
Frequently the downstream detection part and, in particular, the image recording detector element are also laterally displaced synchronously with the phase-shifting structure, for example in a fashion implemented by a design with a motionally rigid coupling of the phase-shifting structure and detector element. This fixed coupling permits a relatively compact design of the wavefront-measuring interferometer part. Particularly for this type of system with motionally rigid coupling of the phase-shifting structure and detector element, however, when use is made of the method, conventional for this purpose of evaluating the wavefront interoferograms, it is observed that there is a limitation of the accuracy which can be achieved for the wavefront measurement, and this is to be ascribed to the fact that the image of the pupil of the imaging system to be measured migrates during the measurement operation in the detection plane of the detector element in conjunction with the synchronous lateral movement of the phase-shifting structure and detector element. This is the case, specifically, with systems which do not use a sine corrected imaging optical system between the phase-shifting structure and the detector element, and holds both for the abovementioned slow lateral movement in the direction to be measured, and for the fast movement in the direction, orthogonal thereto, for suppressing the undesired interference. The pupil migration also occurs when the object-side mask structure is laterally displaced, while the detector element remains undisplaced, and leads with the conventional evaluation methods to a spatial “blurring” of the measured wavefronts, and thus to a so-called “crosstalk” between different Zernike coefficients, in particular Zernike coefficients with large radial powers are underweighted.
The technical problem on which the invention is based is to provide a device and a method of the type mentioned at the beginning which specifically permits comparatively accurate wavefront measurement of an optical imaging system even when the pupil image of the measured imaging system migrates on the detection plane of the detector element owing to a coupled lateral movement of the phase-shifting structure and detector element, or a lateral movement of an object-side mask relative to the detector element.